It will vary depending on the output you want to get from the amp, as well as the voltage comming into the amplifier. Set your gain properly by using a DMM (or idealy an O-scope). Read the gain setting tutorial in the amplifier section
dude the intelliQ is not the ssf. they call it a high pass filter which is basically what a ssf is.
also full,12,24 is the xover settings. if you plan to use an external xover, say your deck, then set it too full or choose 12 or 24 db and set your xover on your amp to say 60-80 hz. and then set gain
filters have a "quality". this makes the most sense with banpass filters, but also applies to lowpass or highpass filters. any "high quality" filter will have a peak in the response, this is similar to a bass boost. so the subsonic filter can be designed to be a SSF (highpass) and bassboost (quasi-bandpass) filter all in one.